The present invention relates to a support apparatus as a suspension apparatus for an internal combustion engine and the like for dampingly supporting an object on a support member. More particularly, it relates to such a support apparatus which has an improved vibration-absorbing characteristic in which transmission of high-frequency vibrations from the support member to the object can be effectively absorbed.
A typical example of a conventional support or suspension apparatus for a vehicle is shown in FIG. 8. In this FIG., the body 1 of a vehicle displaceably supports four axle shafts 2 (only one is illustrated), and a road wheel 3, being rotatable on the ground or the road, is fixedly mounted on a corresponding axle shaft 2. Each road wheel 3, acting as a support member, supports the vehicle body 1, acting as an object to be supported, through a support apparatus in the form of a well-known suspension apparatus S. The suspension apparatus S illustrated includes a shock absorber 5 such as an oil damper connected between the vehicle body 1 and the axle shaft 2 for absorbing and suppressing vibrations of the vehicle body 1, and a coiled spring 6 disposed under compression between a slidable member and a fixed member of the shock absorber 5.
The operation of the above conventional suspension apparatus S will now be described with reference to FIG. 9 which illustrates the vibration response characteristic of the vehicle body 1.
If the road surface 4, on which the vehicle is travelling, is irregular, the road wheel 3 is caused to move up and down, thus generating vibrations in the vertical direction. The vibrations of the road wheel 3 are transmitted to the vehicle body 1 through the suspension apparatus S, i.e., through the shock absorber 5 and the coiled spring 6. In this case, the vibration response characteristic of the vehicle body 1 in relation to the vibration frequency f due to the road surface irregularities is shown in FIG. 9.
In FIG. 9, the ordinate represents the ratio of the vibration amplitude of the vehicle body 1 to the magnitude or height of the road surface irregularities, and the abscissa represents the vibration frequency f of the road wheel 3. Also, the broken line, the solid line and the chain-dotted line represent different characteristic curves which are obtained by changing the damping force of the shock absorber 5 into a low, a medium and a high level, respectively. In addition, f.sub.1, f.sub.2 and A indicate the resonance frequency of the sprung portion or the vehicle body 1, the resonance frequency of the unsprung portion or the road wheel 3, and the discomfortable range of the vibration frequency in which the driver or passenger of the vehicle feels discomfort, respectively.
From FIG. 9, the following facts are shown.
(I) When the damping force of the shock absorber 5 is low or small, the vibration response of the vehicle body 1 (i.e., vibration of the vehicle body 1 responsive to that of the road wheel 3) is low in the discomfortable range A, but the amplitude of vibration of the vehicle body 1 at the resonance frequency f.sub.1 is great, as shown by the broken line. As a result, if the road wheel 3 rides on a projection or the like on the road, the resultant vibrations of the vehicle body 1, which ordinarily have vibration frequencies near the resonance frequency f.sub.1, are not attenuated or converged by the shock absorber 5 in a short time, so the attitude of the vehicle body 1 remains unstable for a relatively longer period of time, impairing the riding comfort. PA0 (II) When the damping force of the shock absorber 5 is great, there is substantially no resonance point in the vibration frequency so that the attenuation or convergence of vibrations of the vehicle body 1 in the relatively low frequency range is materially improved. In this case, however, the vibration response of the vehicle body 1 in the discomfortable range A increases, thus resulting in decreased riding comfort. PA0 1) The damping force is set to a high level if the vibration frequency f is less than the discomfortable range A (f&lt;A). PA0 2) The damping force is set to a low level if the vibration frequency f falls within the discomfortable range A (f=A). PA0 3) The damping force is set to a medium level if the vibration frequency f is greater than the discomfortable range A (f&gt;A).
Accordingly, in view of the riding comfort and the running stability of the vehicle, a compromise is made so that the damping force of the shock absorber 5 is set to be at a medium level between the small and the large level (I) and (II), as shown by the solid line in FIG. 9.
Next, the strength of the coiled spring 6 is considered. For example, a pitching moment will be applied to the vehicle body 1 as when the vehicle is braked, decelerated, or accelerated, and a rolling moment will be applied to the vehicle body 1 as when the vehicle is steered to turn. On these occasions, in order to maintain the attitude of the vehicle body 1 unchanged or at level against such a pitching and/or rolling moment, it is necessary to stiffen the coiled spring 6, i.e., to increase its spring constant k. However, as the spring constant k becomes greater, the resonance frequency f.sub.1 of the vehicle body 1 approaches the discomfort range A, thus impairing the riding comfort.
Consequently, for the purpose of coping with the above-described problem of reduced riding comfort, it is considered that the spring constant k of the coiled spring 6 be decreased and at the same time the damping force of the shock absorber 5 be set in such a manner as to change along the lowermost line in FIG. 9, i.e., to satisfy the following conditions:
It is, however, extremely difficult to change the damping force of the hydraulic shock absorber 5 such as an oil damper in the above manner depending upon the vibration frequency f because of the hydraulic nature thereof.
Furthermore, in order to decrease the strength of the coiled spring 6, some countermeasure must be taken for stabilizing the attitude of the vehicle body 1. To this end, for example, the coiled spring 6 may be replaced by a known pneumatic spring or the like, the spring constant k of which can be changed by controlling the quantity and/or flow of gas such as air filled therein. In this case, however, the response of the pneumatic spring is poor.
Moreover, it is considered that the shock absorber 5 is replaced by a hydraulic actuator, and that the thrust or operating force of the hydraulic actuator is controlled such that the cushioning force of the coiled spring 6 acting against the high-frequency vibrations of the unsprung portion or the road wheel 3 is offset by the hydraulic actuator while maintaining the height or attitude of the vehicle at a constant target level. In this case, however, a hydraulic actuator is generally superior in response to a pneumatic actuator, but a hydraulic fluid employed therewith is non-compressible, so there is the drawback that high-frequency vibrations of the road wheel 3 tend to be directly transmitted through the hydraulic actuator to the vehicle body 1.
Thus, the above-described conventional suspension apparatues generally have the problem that the vibration-absorbing capacity is relatively low, so vehicles equipped with such suspension apparatuses are poor in riding comfort.